1. Field of the Invention
The present invention relates generally to noise masking systems and more particularly to a noise masking system for extending masking noise into a room by vibrating a structural member thereof.
2. Description of the Prior Art
Noise masking systems installed for use in buildings, rooms, large open spaces, etc., are conventionally in use for providing a noise background containing a proper distribution of frequencies and amplitudes that effectively reduce interference associated with ambient background sound within the room. See, for example, W. J. Cavanaugh, et al, "Speech Privacy in Buildings," The Journal of the Acoustical Society of America, Volume 34, No. 4, April, 1962.
Most masking systems use conventional diaphragm speakers which are physically placed within the room at a plurality of predetermined locations. However, structural components such as beams, pipes, air conditioning ducts, etc. generally interfere with the mounting of diaphragm speakers on the ceiling of the room. Such speakers are not mounted on the walls due to the disruption of the aesthetic appearance of the room. In addition, such speakers are not mounted on the floor in the room due to the expense of usable floor space, and for other reasons.
None of the prior art masking systems use vibration of a structural member to inject the masking noise into the room. Prior art transducers which directly attach to the wall so that the wall is appropriately vibrated to a music or sound source, however, are known. Examples of such approaches are found in the patents issued to R. T. Cozart, U.S. Pat. No. 3,636,281 issued on Jan. 18, 1972; A. Komatsu, U.S. Pat. No. 3,728,497 issued on Apr. 17, 1973; and J. F. Cain, U.S. Pat. No. 3,311,712, issued on Mar. 28, 1967. While these prior art approaches generally produce omnidirectional sound emanating from the vibrated structural member, such prior art approaches are expensive to manufacture, not reliable in the field, generally require a great amount of energy to produce the desired sound transmission and exhibit the possibility of rattling at high or low frequencies or at high power conditions. Such conventional approaches, therefore, are generally not applicable to thick or massive structural members. For those types of members, the transducer body itself begins to vibrate excessively without transferring the vibratory energy thru the structural member.
In addition, due entirely to the nature of the structural member (such as a wall), a smooth frequency response is usually not obtainable over the normal frequency range of hearing. For example, in the patent issued to J. F. Cain, supra it is specifically mentioned that two units are desirable in order to provide the best frequency response over the normal hearing range. The above prior art transducers are not suitable for use in noise masking systems since noise masking systems operate day in and day out for the better part of each day thereby imparting considerable wear stress to the several components of the transducers.
The present invention avoids the problem of the prior art approaches in that a spectrum shaper is utilized to establish a desired sound distribution within the room by compensating for the particular transmission characteristics of the structural member being vibrated. The spectrum shaper of the present invention is adjusted to provide a substantially smooth response of masking noise within the room of desired spectrum shape. In addition, a conventional diaphragm speaker is utilized to impart vibration to the structural member having either a high or low mass. The use of conventional diaphragm speakers, as the vibration exciting transducer, results in a lower cost system exhibiting greater wear capabilities.